The formation of veins and their microstructures

 

Paul D. Bons

Epsilon Earth Processes Simulation Laboratory, Department of Earth Sciences,Monash University, Clayton (Melbourne), VIC 3168, AUSTRALIA

E-mail: bons@mail.uni-mainz.de

Table of contents

Abstract1. Introduction2. Terminology2.1. macroscopic morphology2.1.1. Pressure fringes2.1.2. General veins2.2. Microscopic morphology2.2.1. Blocky texture2.2.2. Elongate blocky texture2.2.3. Fibrous texture2.2.4. Stretched crystals2.2.5. Combinations of textures2.2.6. Partially filled veins2.3. Growth morphology2.3.1. Syntaxial veins2.3.2. Antitaxial veins2.3.3. Composite veins2.3.4. Syntaxial & antitaxial pressure fringes2.3.5. Ataxial or stretched veins2.3.6. A-syn-anti-bi-uni-taxial?2.3.7. Vein morphology and structural analysis

3. Vein formation3.1. transport mechanisms3.1.1. Transport through a fluid:diffusional flow3.1.2. Transport with a fluid: fluid flow3.1.3. Darcian advective fluid flow3.1.4. Mobile hydrofractures3.2. Precipitation in a vein3.2.1. The crack-seal mechanism3.2.2. Vein growth without fracturing3.2.3. Vein growth in fractures3.2.4. Coupled fracturing and fluid flow4. ConclusionsAcknowledgmentsAppendix A. Darcian flow and mobilehydrofracturesAppendix B. Crack-sealmicrostructuresReferences

Abstract

 

An overview is presented of the various vein types, their microstructures and the processesthat lead to vein formation. Vein types and their structures are divided into three categories:

1. macroscopic morphology (e.g. sigmoidal vein),

2. microscopic morphology (e.g. fibrous, blocky, etc.),

3. growth morphology (e.g. syntaxial, antitaxial, etc.).

The formation of veins involves two steps: (a) transport of vein forming material (nutrients)to a vein and (b) precipitation of the vein forming mineral(s). Main modes of transport arediffusional transport, advective or Darcian fluid flow and mobile hydrofractures. Causes forprecipitation range from local supersaturation in, for instance, pressure shadows, which ismostly associated with diffusional transport to, often large, supersaturation in externallyderived fluids. Variations in fluid pressure between hydrostatic and lithostatic can also causeprecipitation of vein material.

In general, fibrous textures form due to diffusional transport to low pressure sites, such aspressure shadows. Fibrous textures can form without brittle fracturing. Elongate blocky andstretched crystal textures form in case of repeated fracturing and sealing (crack-sealmechanism). Nutrient transport can be by diffusion or by advective fluid flow. Rapid fluidflow, especially in mobile hydrofractures, can bring fluids quickly from their source region tothe sites of vein formation, allowing large supersaturation and precipitation of massiveamounts of vein material. Resulting veins are often blocky, although elongate blocky /stretched crystal textures can also be found if repeated crack-sealing occurs.

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1.0 Introduction

 

Veins occur in many forms, are composed of many different minerals and occur at all levelsof the Earth's crust and mantle. Their morphology, petrology and chemistry is a valuablesource of information in a range of geological disciplines. The association of many oredeposits (particularly gold) with veins makes them even more relevant to geology. It istherefore not surprising that veins have been studied extensively. Yet, the formation of veinsis still not fully understood. One surprising aspect of veins is that no one has been able tosuccessfully and consistently simulate the formation of the variety of vein types - not in realrocks, nor in rock analogues. Attempts with varying degrees of success have certainly beenmade (Post 1989, Li & Means 1995, Means & Li 1995a&b, Bons & Jessell 1997, andprobably many more which never have been published).(Fig. 1)

Figure 1. Vein like textures in a thin sheet of octachloropropane (OCP, C3Cl8) between glassplates. OCP is an organic material, that has been used to simulate microstructuraldevelopments in crystal-plastically deforming rocks (Means 1989). Here, a sheet offine-grained OCP was heated on a hot plate to its melting point (160°C) and immediatelytaken off when melting started, to prevent melting of the whole sample. Sliding of the topglass plate relative to the lower plate, caused extension between different parts of the partiallymolten sample, with immediate crystallisation out of melt occurring within the extensionzones, resulting in vein-like structures. The whole process took place in a few seconds. (a)Whole "vein" with a curved stretch-crystal type texture. Width of view 5 mm, crossed polars.(b) Detail of a "vein" with an elongate blocky / stretched crystal texture and serrate("radiator") grain boundaries, typical for stretched crystals, but possibly due torecrystallisation. Width of view 1.2 mm, crossed polars.

It is a pleasure to be able to state in this volume, that probably the most interesting andreal-life-looking vein simulations were done under the supervision of Win Means (Li &Means 1995, Means & Li 1995a&b). However, these experiments and those by others, havenot yet resulted in full understanding of the formation of veins. This paper will thereforefocus on what is known about vein formation. First, the various types of veins and their(micro-) structures are discussed. Secondly, the modes of transport of vein forming materialare reviewed, followed by the third and last section that deals with the processes andcircumstances that lead to precipitation of vein forming minerals. The processes that lead tovein formation are linked with the structures that are found in veins. It is impossible to go intodetail of each individual aspect of vein formation within the limited space of this paper. Sometopics are therefore only dealt with briefly, especially when much literature on the topic isavailable (e.g. veins and fluid flow through fractures). Other topics are discussed in moredetail, especially if relatively little is published on these topics, such as the two end members

of vein forming processes: vein growth without fracturing and veins formed out of mobilehydrofractures.

Before proceeding to discuss the formation of veins, it is important to define veins. In thispaper, I define veins as "distinct polycrystalline mineral volumes that formed within a rockand that are filled with one or more minerals that precipitated from an aqueous fluid". Theterm "polycrystalline" figures in this definition to exclude individual metamorphicporphyroblasts. "Formed within a rock" is added to exclude evaporitic precipitates, althoughsuch precipitates can also form within sediments. Definitions of veins often include adescription of the shape of veins, typically planar or lenticular. I do not do so, as veins havemany shapes and I specifically want to classify pressure fringes as veins. Finally, smalligneous bodies are also often termed "veins", but these are excluded in the definition givenhere as their melt origin is different from that of the veins discussed in this paper. 

One could say that veins are structures that reveal a deformation history for structuralgeologists (e.g. Ramsay & Huber 1983), while veins are principally indicators of past fluidflow for metamorphic petrologists and geochemists (e.g. Thompson 1997). This difference inapproach has not helped the issue. We will only achieve real understanding of veins if wecombine all of these aspects and also if we consider the full range of vein-forming processesand the resulting vein types. In this paper I review the terminology of veins (traditionallymostly the domain of structural geologists) and the processes that lead to vein formation(more the field of metamorphic petrologists), with the aim of clarifying how different veintypes form and how to recognise the processes that led to vein formation.

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2.0 Terminology

 

A proper terminology is very important to describe the many shapes of veins and the manydifferent microstructures within veins. The terminology for veins that is currently in use, ismostly derived from Ramsay & Huber (1983) and Passchier & Trouw (1996). I use thisterminology, with only minor refinements. Terms for the description of veins can be groupedin three categories. the first two relate to the structure of veins at different length scales,namely:

- macroscopic morphology (e.g. sigmoidal vein)

- microscopic morphology (e.g. fibrous)

The third category, termed "growth morphology" here, relates to the symmetry of thestructures (e.g. antitaxial).

Ideally, the geometry and structure of a vein can be fully described by three terms, one fromeach category (e.g. antitaxial fibrous sigmoidal vein). We will see that not all combinations ofterms occur and that some veins have a combination of features (e.g. partly blocky and partlyfibrous).

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2.1 Macroscopic MorphologyTerms relating to macroscopic morphology, i.e. the shape of veins, are the least consistent, orwell defined. Broadly speaking, we can divide veins in two categories:

a) veins that are directly related to some hard object as are pressure fringes (Fig. 2)

b) veins with shapes that are not primarily defined by a relatively hard object, but by fracturesor other factors. 

Figure 2. Pressure fringe of fibrous quartz around a concretionof iron ore in a BIF-chert from the Hamersley ore province,Pilbara, West Australia. Width of view 2.3 mm, crossedpolars. 

2.1.1. Pressure fringes

Pressure fringes are veins that form on the two low pressure sides of hard objects, usually oreminerals, but also other objects, such as crinoid stems (Durney & Ramsay 1973, Ramsay &Huber 1983, Selkman 1983, Beutner & Diegel 1985, Etchecopar & Malavieille 1987, Aerden1996). They are termed pressure fringes if they have sharp edges and usually also if theirinternal structure (see below) is fibrous (Fig. 2). If not, they are termed pressure shadows(Fig. 3), which usually have diffuse boundaries and not a fibrous internal structure. Oneshould however note, that recrystallisation can destroy the high grain boundary energy fibrousinternal structure of a pressure fringe, making it appear like a pressure shadow. 

Figure 3. Quartz + mica pressure shadow adjacent to a quartzporphyroclast (on right, grey grain with inclusions) in aquartz-mica schist from Nooldoonooldoona Waterhole, S.W.Mount Painter Inlier, Arkaroola, South Australia. Width ofview 3.2 mm, crossed polars. Note the sharp boundary of thepressure fringe (Fig. 2) in contrast to the vague boundary ofthe pressure shadow.

2.1.2. General veins

Most veins have the shape of lenses, tabular bodies or blobs. A variety of names (especiallyin mining; e.g. Barton 1991, Dong et al. 1995) exist for different shapes and positions withinthe rock. It is impossible to go into details here of every name or term that has been used inthe literature, instead, I delineate three broad categories: tension veins, shear veins andbreccia veins.

Many veins have their shape and orientation determined by structures such as fractures, faults

 

or bedding (Fig. 4). The formation of fractures is favoured by high fluid pressures (Pf), as thedifferential stress needed to create fractures is reduced by high fluid pressures (Fig. 5). Atsufficiently high differential stress ( ∆σ = σ 1 - σ 3) and Pf, shear fractures form at angles lessthan 45° to the maximum stress (Fig. 5.c). Although some dilation must occur on slip alongsuch fractures, the main mode of displacement is parallel to the fracture plane and suchfractures provide limited space for vein formation. If the fluid pressure is very high (Pf > σ 3 +T, T = tensile strength), extensional fractures can form where the main mode of displacementis normal to the fracture plane (Fig. 5.b) (Secor 1965). Such fractures provide more space forvein minerals to grow into and indeed, many veins appear to have grown in such extensionalfractures (tension gashes). As tensional fractures provide the best opportunity for veinformation, it is not surprising that "tension veins", also called "tension gashes", "tensionfissures" or "gash-veins" are common (Ramsay & Huber 1983, Rickard & Rixon 1983).These veins are usually lenticular is shape. Their size can range form mm-size to kilometres(Hippertt & Massucatto 1998) (Fig. 6). As can be inferred from Fig. 5.b, the formation oftensional fractures not only requires a high fluid pressure, but also limits the maximumpossible differential stress (Etheridge 1983). Tensile strengths of rocks are generally in theorder of 10 MPa, with values reaching several tens of Mpa at the most (Lockner 1995). Thislimits the differential stress during tensile fracturing to usually about 20-40 MPa. 

Figure 4. Fibrous antitaxial calcite veins in carbonaceousshales. The veins form fracture-like sets with side splayswhere the interaction between two veins caused a localdisturbance of the stress field. Tapley Hill Formation,Opaminda Creek, Arkaroola, South Australia.

Figure 5. Mohr failure envelope representation (after Lockner 1995) in agraph of the shear stress (τ) against principal normal stresses (σ1 and σ3,with the rock pressure Pr = (σ1+σ3)/2). The Mohr-circle is a circle withits centre on the horizontal axis. Points on the circle represent normalstress / shear stress conditions on all possible planes. The angle β is theangle between the failure surface and the direction of maximum principlestress (σ1). (a) When the Mohr-circle does not touch the Mohr-envelope,stresses are too low to induce failure. (b) Fluid pressure (Pf) reduces theeffective pressure, which moves the Mohr circle to the left. Fluidpressure can rise until the Mohr-circle touches the Mohr envelope, atwhich point failure occurs. In the given case, with Pf>σ3, failure is tensile(β=0°). (c) Shear failure (β=40°) can occur at relatively lower fluidpressure, but at a higher differential stress. (d) Tensile strength of anexisting fracture is close to zero, which shifts the Mohr-envelope down.Less differential stress and fluid pressure is needed to reactivate such anexisting fracture. 

Figure 6. Massive tension vein on Poolamacca Station,Broken Hill Inlier, New South Wales (Australia). The vein isapproximately 50 metres wide and a few hundred metreslong and consists of pure milky white quartz. 

Tension veins are often found in en échelon arrays (Fig 7.a). In such arrays they often have asigmoidal (S or Z) shape (Durney & Ramsay 1973, Hanmer 1982, Rickard & Rixon 1983,Selkman 1983, Nicholson 1991, Olson & Pollard 1991, Passchier & Trouw 1996, Becker &Gross 1999, Smith 1999). The classical interpretation of such arrays is simple shearingparallel to the vein array in the direction opposite the way the vein tips point. The veinsoriginally formed parallel to the maximum shortening direction (135°) and subsequentlyrotate. Vein propagation remains in the 135° direction, resulting in the development of thesigmoidal shape (Fig. 7.b). New veins may form cutting existing ones and these veins alsoinitially form in the 135° direction. Continuing deformation at the en échelon array andformation of new veins in the deformed zone may eventually lead to the formation of onethrough-going vein (Fig. 8) (e.g. Wilkinson & Johnston 1996).  

a

b

Figure 7. (a) En échelon array of veins in a dextral simple shear,resulting in a Z-shape of the tension veins. (b) Movie showing thedevelopment of a sigmoidal tension vein in sinistral shear. See textfor discussion.

Figure 8. Set of sigmoidal en échelon veins that have amalgamated into a singledextral shear vein. Heavitree Quartzite, Ormiston Gorge, Central Australia.Photograph courtesy Alice Post.

Whereas tension veins tend to have at least their initial displacement direction normal to thefracture surface, fault related veins show evidence for a dominant fault-parallel displacement.Even then, some dilation is needed to provide space for vein growth. Slickenfibres (Passchier& Trouw 1996) occur on shear fractures (Fig. 9), but most vein growth is usually found onmore dilatant pull-aparts (Peacock & Sanderson 1995, Brown & Bruhn 1996). Shear fracturescan form, in intact rocks, at lower Pf than for tensional fractures, but a higher differentialstress is needed (Fig. 5.c). However, existing planes of weakness (faults, bedding contacts)can reduce the tensional strength and hence the necessary differential stress and fluid pressureneeded to induce fracturing (Cox & Paterson 1989, Sibson & Scott 1998) (Fig. 5.d). Veinsthus tend to form along fault planes or parallel to bedding and there particularly in structuressuch as folds (e.g. "saddle reefs") and releasing bends (Raybould 1975, Cox et al. 1986,Henderson et al. 1990, Cosgrove 1993, Jessell et al. 1994, Glen 1995, Windth 1995, Fowler1996, Fowler & Winsor 1997, MacKinnon et al. 1997). Another case where mechanicalheterogeneities play an important role in the shape of veins is that of veins in boudinagenecks (Lohest 1909, Cloos 1947, Platt & Vissers 1980, Mullenax & Gray 1984, Malavieille &Lacassin 1988, Smith, 1998).  

Figure 9. Photograph looking down on slickenfibres in HeavitreeQuartzite (Ormiston Gorge, Central Australia). diameter 1 A$ coinapprox. 2 cm. Photograph courtesy Alice Post.

Breccia or net veins form a matrix between clasts in a breccia (Fig. 10). These typically occurin hydrothermal (ore) deposits. True breccia veins formed in one event of extensivefracturing, without significant preferred orientation. However, abundant veining of othertypes and/or the activity of multiple veining events with different cross-cutting orientationsmay produce breccia-like veins (Valenta et al. 1994). 

Figure 10. Hydrothermal breccia of altered (haematised and silicified) wallrock pieces in a matrix of white quartz. Hammer on right edge of photo forscale. Mt. Gee, Arkaroola, South Australia.

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2.2 Microscopic MorphologyThe microscopic morphology relates to the texture or the shape and arrangement of crystalsinside a vein. Here I distinguish four primary categories:1. Blocky2. Elongate blocky3. Fibrous4. Stretched

This list is not exhaustive: especially shallow hydrothermal systems can produce a variety oftextures and the reader is referred to Dong et al. (1995) for a review.

2.2.1. Blocky texture

A blocky texture is a texture in which grains are roughly equidimensional and randomlyoriented. The texture in most granites could for instance be termed 'blocky'. Blocky texturescan be primary, if, during vein growth, nucleation of new grains continues. Blocky texturescan, however, also be secondary and due to recrystallisation of a primary texture.

2.2.2. Elongate blocky texture

Crystals in an elongate blocky texture (Fisher & Brantley 1992) are typically moderatelyelongate (length/width ratio generally in the order of 10) and the long axes of crystals arealigned (Fig. 11). This texture forms when nucleation of new grains does not occur duringvein growth, and all growth is by crystallographically continuous overgrowths on existinggrains and growth occurs at the tips of existing crystals. The 'seed grains' can be pre-existinggrains in the wall rock of a vein, or grains formed during an initial nucleation stage. Elongateblocky textures show evidence for crystallographically controlled growth competitionbetween grains (Mügge 1928). Crystals growing into a fluid typically show facetedmorphologies as some crystal faces grow faster than others. Some grains, which arecrystallographically oriented favourably with respect to the general growth direction, willoutgrow unfavourably oriented grains. The faster growing 'winner' grains not only growfaster, but also wider, at the expense of the 'looser' grains. This leads to a gradual increase ingrain width in the growth direction and the development of a crystallographically preferredorientation for the 'winner' grains (Mügge 1928, Cox & Etheridge 1983) (see Appendix B). 

Figure 11. Photomicrograph of elongate blocky texture in aquartz vein. Vein crystals grew out from quartz grains in thesandstone wall rock (below), towards the centre of the vein(top). Growth competition reduced the number of grains awayfrom the vein margin. Approximately horizontal dust and fluidinclusion trails suggest step wise crack-seal growth. FoldedPalaeozoic turbidites, East Gippsland, Victoria (Australia).Width of view 8 mm, crossed polars.

2.2.3. Fibrous texture

In a fibrous texture, the rod-shaped grains can achieve a much higher length/width ratio than

 

in elongate blocky textures (Fig. 12). As in an elongate blocky texture, the grains’ long axesare aligned. The distinguishing feature is that fibrous veins hardly show any growthcompetition. All grains have approximately the same shape. As with elongate blocky texture,a fibrous texture can only develop if no nucleation takes place after growth started. 

Figure 12. Photomicrograph of an antitaxial fibrous calcitevein. Fibre growth was outward from the median line, which ismarked by a string of inclusions of the calcareous shale hostrock. Outward growth can be determined by the slight increasein average fibre width away from the median line. Smallblade-like quartz crystals precipitated at the vein margin,possibly onto existing small quartz grains in the shale. TapleyHill Formation, Opaminda Creek, Arkaroola, South Australia.Width of view 4 mm, crossed polars.

It should be noted here, that I support the distinction between elongate blocky veins andfibrous veins as used by Fisher & Brantley (1992). This distinction is currently not usuallymade by other workers, who tend to call both categories 'fibrous'. However, until theseventies, it was recognised that the two are different (Durney & Ramsay 1973). Thepopularity of the 'crack-seal' mechanism, first proposed in the paradigmatic paper by Ramsay(1980) is perhaps the cause for the grouping together of the two categories. Although acrack-seal origin for all "fibrous veins" is favoured by some (e.g. Cox & Etheridge 1983, Cox1987, Urai et al. 1991), different vein forming mechanisms may operate and therefore adistinction in fibrous and elongate blocky textures should be made.

2.2.4. Stretched crystals

In the previous textures, additional vein material formed by precipitation on the surface ofexisting grains. The primary distinction between the previous textures and stretched crystalsis that in stretched crystals, additional growth took place inside the grains (on the surfaces ofthe half grains), with the space for new-growth provided by (micro-) fractures that cutthrough the grains (Fig. 13). Fluid inclusions, dust rims or cathodo-luminescence images mayreveal this. Stretched crystals often have jagged boundaries ("radiator" structure) andsometimes the two halves of the original grain can still be recognised at both ends of astretched crystal. 

a

b

Figure 13. Photomicrograph of stretched quartz crystals in asandstone hosted vein in (a) plane polarised light and (b) crosspolarised light. The vein material was formed by addingmaterial to existing grains in the sandstone. White arrowsindicate parts of a stretched vein crystal that formed twohalves of one grain before the vein formed. Vein wall paralleldust trails and wall rock inclusions (black arrows) indicate thatthis vein formed by repeated crack-sealing, which can also

produce characteristic serrated grain boundaries ("radiator"structure, red arrows). Folded Palaeozoic turbidites, EastGippsland, Victoria (Australia). Width of view 2.3 mm.

2.2.5. Combinations of textures

Not all veins display only one texture. It is not uncommon for veins to be partly fibrous andpartly (elongate) blocky as in figure 14. 'Polytextured' could be a possible term for such veins,but it is more important that the different textures for such a vein are described than to definea new and less meaningful single term for the many different possible combinations oftextures. Two types of polytextured veins can be distinguished: (1) sequential growth ofdifferent textures as in fig. 14a&b, where first one texture forms and then another one, and (2)simultaneous growth of different textures at different sites within one vein (Fig. 14c). Veinsof the second type occur in at Opaminda Creek, Arkaroola, when veins cut shales andcarbonaceous silt stone layers. Fibrous textures form in the shale, but stretched crystalsdevelop where the vein transects silt stone. Different mechanical properties probably play arole here, with fractures only forming in the silt stone layers and vein sections in the shalegrowing without any fracturing (see Ch. 3.2.2). 

a

b

c

Figure 14. Photomicrograph of a polytextured, fibrous andblocky calcite vein in (a) plane polarised light and (b) crosspolarised light. The initial veins is marked by a thin line ofwall rock inclusions (S) and a line of quartz crystals (Q).Subsequently, two stages of vein growth occurred: (1)antitaxial fibrous growth towards the left and (2) open cavitygrowth on the right. The cavity infill took place byovergrowth of the first vein material (at line Q), but also bynucleation and growth of new crystals, resulting in adominantly blocky texture. Infill of the cavity was notcomplete as cavities with faceted grain surfaces remained(C). This indicates that the blocky growth was probably thelast growth event, post-dating leftward antitaxial fibrousgrowth from line S (right half of vein not shown). Width ofview 10 mm. (c) Antitaxial fibrous calcite vein in shale(right) grading into stretched calcite in silt layer (left).Crossed polars; width of view 12.5 mm. Tapley HillFormation, Opaminda Creek, Arkaroola, South Australia..

2.2.6. Partially filled veins

Veins may contain voids or cavities. Such cavities may be a result of incomplete filling of thevein. One form of incomplete filling is where a continuous crust of crystals lines the wallrock, with the vein crystals often having euhedral crystal faces facing the remaining cavity.Another form is where individual crystals span the entire vein width, but have open space inbetween (Henderson et al. 1990). These voids can later be filled by side-ways overgrowth ofthe first crystals (Fig. 15). If the vein is completely filled, the original existence of such voidsis sometimes only visible with cathodo-luminescence. 

Figure 15. Photomicrograph of quartz vein with a voidbetween elongate crystals. Crystals grew from left to right inmany increment as indicated by repeated vertical dust trails.The void between crystals A and B is being partially filled byside-ways growth (arrows) of grains A and B. FoldedPalaeozoic turbidites, East Gippsland, Victoria (Australia).Width of view 3.2 mm, plane polarised light. 

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2.3 Growth MorphologyThe third class of terms that are commonly used for veins is grouped here under the name'growth morphology'. Growth morphology terms relate to where the site(s) of progressivegrowth are located in the vein, which determines the direction of growth of the vein formingcrystals. 

2.3.1. Syntaxial veins

In syntaxial veins (Durney & Ramsay 1973) (Fig. 16.a), growth occurs on a single plane: themedian plane. On this plane, usually a thin fracture, material is added by overgrowth on veincrystals on both sides of the growth plane. Hence, the latest precipitated vein material islocated at the median plane, while the first and oldest precipitate is found at the outside of thevein: at the vein - wall rock contact. Syntaxial veins can be symmetric, but often the growthplane is closer to one side of the vein, which produces a marked asymmetry in the vein (Fig.16.b) (Fisher & Byrne 1990, Fisher & Brantley 1992). Syntaxial growth usually occurs whenthe vein forming mineral is a major constituent of the host rock. Vein crystals then growepitaxially off grains in the wall rock. 

a 

b 

Figure 16. (a) Movie illustrating the growth of a syntaxial vein. Thesingle growth surface is marked by a flashing white line. A record ofthe oldest part of the opening history can be found at the veinmargins (horizontal opening, normal to the vein), while evidence forthe youngest oblique opening direction is found in the centre. (b)Movie of the growth of a one-sided asymmetric vein, where growthis completely on one side of the vein.

2.3.2. Antitaxial veins

When the vein forming mineral is not a major constituent of the host rock, antitaxial veins(Durney & Ramsay 1973) commonly form (Fig. 12 & 17). In antitaxial veins there are twogrowth surfaces: one on each outer surface of the vein, between vein and wall rock. New veinmaterial is added simultaneously at both these surfaces (Fig. 18). Hence, the youngestmaterial is located at the outside of the vein, while the first precipitate is situated in themiddle, on the median plane. The median plane in an antitaxial vein is clearly of a differentnature than the median plane of a syntaxial vein. In antitaxial veins, which are usually(always?) fibrous, the median plane is marked by a string of wall rock inclusions (Fig. 12 &17) or by a thin zone of differently textured vein material. Figure 19 shows a case where azone of elongate blocky textured calcite occurs in the middle of a fibrous antitaxial calcite

vein.  

Figure 17. Tips of two parallel en échelon antitaxial fibrouscalcite veins. Mean fibre width increases slightly from themedian line outwards, which indicates that growth wasoutwards (antitaxial). Fibre shape is symmetric around themedian line, except near the tips. Growth and propagation ofthe veins caused bending if the shale "bridge" in between.Tapley Hill Formation, Opaminda Creek, Arkaroola, SouthAustralia. Width of view 13 mm, crossed polars.

Figure 18. Movie illustrating growth of an antitaxial fibrous vein,which typically contains a mineral different from those dominant in thewall rock. Growth occurs on the two outer margins of the vein. Arecord of the oldest part of the opening history can be found at the veincentre (horizontal opening, normal to the vein), while evidence for theyoungest oblique opening direction is found in the margins. Note thatnot all crystals keep growing all throughout the growth history, butcompetition is usually not strong in antitaxial fibrous veins.

Figure 19. Median zone in a dominantly fibrous antitaxialcalcite vein. The median zone has an elongate blocky texturewith growth mainly from the right to the left. Small wall rockinclusions and quartz crystals mark the edge of the medianzone. Antitaxial growth took place after formation of themedian zone, with fibres growing out of median zone grains.The fibre width is much smaller than the grain size in themedian zone, but comparable in size to the deformation twinsin these grains. This indicates that the initial (median zone)vein was deformed before the onset of fibre growth. TapleyHill Formation, Opaminda Creek, Arkaroola, South Australia.Width of view 6.4 mm, crossed polars.

2.3.3. Composite veins

If a vein is composed of two minerals (e.g. quartz and calcite), it can occur that one mineralshows syntaxial growth, while the other grows simultaneously with an antitaxial growthmorphology. The syntaxial mineral started growing from the wall rock inwards, while thesecond mineral grew from the inside outwards. In this case there are two growth surfaces.Such growth morphologies are called "composite" by Durney and Ramsay (1973). It issuggested here to also use the same term for pressure fringes, and not "complex fringes" asused by Passchier and Trouw (1996). The term composite should be reserved for veins whereboth morphologies and minerals occupy similar proportions of the vein. Many antitaxialcalcite veins, for instance, have a thin rim of syntaxially grown quartz crystals (Fig. 12 & 17)(Williams & Urai 1987, Urai et al. 1991), but these rims are too thin compared to the vein towarrant classification as composite veins.

2.3.4. Syntaxial & antitaxial pressure fringes

 

The terms syntaxial and antitaxial are also used for pressure fringes (Durney & Ramsay 1973)(Fig. 20). The convention for pressure fringes is somewhat confusing as it seems to beinconsistent with the convention for veins. 'Syntaxial' is a term from crystallography anddenotes overgrowth on a crystal in crystallographic and mineralogical continuity. Syntaxialveins are called such, as these veins usually form by crystallographically syntaxialovergrowth of wall rock grains. 'Antitaxial' (not a crystallographic term) then signifies theopposite. In an antitaxial veins, the new precipitate is not in crystallographic or mineralogicalcontinuity with wall rock grains. Instead, growth occurs by crystallographically syntaxialovergrowth of vein crystals. The terms syntaxial and antitaxial are therefore defined withrespect to a reference material: the wall rock in case of veins. 

Figure 20. As withveins, thedifference betweensyntaxial andantitaxial pressurefringes isdetermined by theposition of thegrowth surface.Syntaxial fringes(crinoid-type) havetheir growthsurface betweenfringe and wallrock, while themore commonantitaxial orpyrite-type fringeshave the growthsurface between theobject and thefringe.

It is somewhat confusing at first sight that for pressure fringes, the object was chosen as thereference material and not the wall rock. In a syntaxial pressure fringes, growth occurs on theoutside of the object + fringe system and the fringe crystals are syntaxially (or sometimesepitaxially) overgrowing the object. Calcite fringes on a crinoid stems are classical examplesof syntaxial pressure fringes (hence the "crinoid-type" of Ramsay & Huber 1983). In anantitaxial pressure fringe, the opposite occurs: the fringe crystals are crystallographically andmineralogically unrelated to the object (and possibly to the wall rock as well). New growthoccurs at the contact between object and fringe, with the object often being pyrite("pyrite-type" of Ramsay & Huber 1983). An excellent overview of the use of pressurefringes for tectonic analysis is given by Passchier & Trouw (1996), who, incidentally, use theterm "strain fringe" instead of "pressure fringe". Also note that they use the term "complexfringes" and not "composite fringes" (which would be consistent with Ramsay & Huber 1983)for fringes that exhibit both antitaxial and syntaxial growth. 

2.3.5. Ataxial or stretched veins

In the previous cases we saw growth on one or two surfaces. These surfaces remained thesame throughout the growth history. A different class of veins is formed when the position ofthe growth surface changes through time (Fig. 21). This happens when a vein forms from asuccession of fractures that fill with vein material. These fractures can cut the host rock andvein at different locations and multiple fractures can be present at any given moment. As suchveins are neither syntaxial, nor antitaxial, the term ataxial is used (Passchier & Trouw 1996).The term 'stretching veins' (Durney & Ramsay 1973) is also appropriate, as the resulting grainmorphology is that of stretched crystals. One can recognise two end-member cases ofstretching veins: one where all fractures occur within the growing vein (Fig. 22.a) and onewhere fractures occur randomly and it becomes difficult to distinguish between a vein and thewall rock, as in fact there are multiple veins (Fig. 22.b).  

Figure 21. Stretched calcite veins in carbonaceous shale. It isclear from the wall rock slivers and thin crack-like veinsoutside the main veins, that the veins grew by many crack-sealevents, each crack being up to a few tens of microns wide. It isdifficult to impossible to determine the sequence of fracturing.Individual stretched crystals span the whole width of veins.Tapley Hill Formation, Opaminda Creek, Arkaroola, SouthAustralia. Width of view 3.8 mm, plane polarised light.

a 

b 

Figure 22. Movies illustrating the formation of stretched veins. (a)Fully localised fracturing end member. All fracturing occurs withinthe growing vein. The margin of the vein is distinct. (b) Completelyrandom position of parallel fractures. A distinction between wallrock and veins is difficult to make. In both movies, opening isinitially horizontal and then oblique. This opening history cannot bedetermined from the shape of the stretched crystals, which onlyreveal the average opening direction over the whole growth history.

2.3.6. A-syn-anti-bi-uni-taxial?

The terms "syntaxial" and "antitaxial" (Durney & Ramsay 1973) gained common use ingeology thanks to popular text books such as Ramsay & Huber (1983) and Passchier &Trouw (1996). Passchier & Trouw (1996) added the term "ataxial" veins for what Ramsay &Huber called "stretching" veins. They also extended the use of syntaxial and antitaxial topressure fringes or strain fringes (see Ch. 2.3.4). More recently, Urai (pers. comm.) andHilgers et al. (in press) proposed additional -taxial terms: "unitaxial" and "bitaxial" (Fig. 23).All these terms may be confusing if they are not fully and accurately defined and consistentlyused by different authors. 

Figure 23.Unitaxial andbitaxial refer tothe directions (redarrow) of growthat the growthplane or accretionplane (blue line).(a) In unitaxialgrowth, growth isin one directiononly at thegrowth plane,which is the casein antitaxial veinswith two unitaxialgrowth planes.(b) Withsyntaxial growth,there is only onegrowth plane onwhich bitaxialgrowth takesplace. (c) Onesided veins havea single unitaxialgrowth planebetween vein andwall rock andcould be termed"unitaxialsyntaxial".

Antitaxial was defined by Durney & Ramsay (1973), Ramsay & Huber (1983) and Passchier& Trouw (1996) for veins in which the crystals inside the vein do grow towards the wall rock,from a median line. In their figures (fig. 13.9 & 13.24 in Ramsay & Huber (1983) and fig. 6.6in Passchier & Trouw (1996)), antitaxial veins have two growth planes, as in Fig. 12, 17 &23a. Syntaxial veins grow from and in continuity with the wall rock inwards. Syntaxial veinshave only one growth plane (Fig. 16.a and 23.b). At the growth surface of an antitaxial vein,growth is in one direction only and hence they can be termed unitaxial, while growth insyntaxial and stretching veins is from both sides of the growth plane and these veins thusshow bitaxial growth.

Whereas the terms "syntaxial", "antitaxial" and "stretching/ataxial" veins refer to the wholevein, the terms "bitaxial" and "unitaxial" refer to growth at one single growth plane. Thisterminology would normally not cause any problems and one may even argue whether theintroduction of the terms "bitaxial" and "unitaxial" is necessary as all antitaxial veins wouldappear to be unitaxial and all syntaxial and ataxial veins are bitaxial. Problems however arisewith completely asymmetric or one-sided veins, that have only one growth plane between the

wall rock and one side of the vein (Fig. 16.b and 23.c). Such veins are described by Fisher &Byrne (1990) and Fisher & Brantly (1992) and the "antitaxial fibrous" veins of Cox (1987)could also be such, although it is not fully clear from his figures or text. Are such veinsantitaxial because growth is towards the wall rock, albeit only on one side of the vein? Or arethey syntaxial, because growth is seeded on the wall rock, albeit only on one side? What isclear, is that these veins are unitaxial, as growth is only in one direction at the growth plane.Ambiguity can be avoided if the terms antitaxial, syntaxial and stretching/ataxial are strictlyused to describe a whole vein and unitaxial and bitaxial for the growth situation at a singlegrowth plane:

- An antitaxial vein has two persistent growth planes on the outer surface of the vein, wheresimultaneous growth occurs unitaxially outwards (Fig. 23.a).

- A syntaxial vein has only one persistent growth plane. Normally growth is bitaxial at thatplane and growth is inwards from the wall rock (Fig. 23.b). However, unitaxial growth canoccur, in which case the growth plane is on one side between the vein and the wall rock andthus only one half of the syntaxial vein develops (Fig. 23.c).

- Stretching or ataxial veins do not have one or two persistent growth planes but alternatingplanes at different sites. Growth at the "jumping" growth plane is bitaxial.

2.3.7. Vein morphology and structural analysis

It is clear that syntaxial, antitaxial and composite veins can provide detailed information onthe progressive sequence of conditions under which these veins formed. These veins areelongate blocky or fibrous and the direction and curvature of the crystals indicate thekinematics of deformation and the orientation of the stress field. Curvature of vein crystals isa result of progressive change of the vein orientation with respect to the stress field aroundthe vein. Such a change in orientation can result from a change in orientation of the stressfield (as for instance in multiple deformation events) and/or a change in orientation of thedeforming vein. The correct growth morphology must be determined for any strain analysis.The direction of growth in fibrous or elongate blocky veins can usually be determined by anincrease in average grain width in that direction. Careful analysis can reveal this (Durney &Ramsay 1973, Ramsay 1980, Winsor 1987, Passchier et al. 1996, Passchier & Urai 1988,Aerden 1996). It is important in such analyses that it is recognised that crystal shapes do notalways fully reflect the displacement path of the vein wall rock, or 'opening trajectory'(Williams & Urai 1987, Urai et al. 1991). A full discussion of the use of veins for structuralanalysis is beyond the scope of this paper, but this point will be discussed in some more detailin Appendix B.

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3.0 Vein Formation

 

Vein formation essentially involves two steps:

A. transport of vein forming material (nutrients) to the growing vein,

B. precipitation of the vein forming mineral(s) in the growing vein.

As veins were defined as forming from precipitation from an aqueous fluid (possiblycontaining CO2, but excluding melt), it is clear that material transport involves transport insolution and that precipitation involves supersaturation of a fluid. This section deals with howand why material can get transported in solution to veins and why at some stage the fluid mayget supersaturated and precipitate vein forming mineral(s). Transport and the cause forprecipitation are by no means always independent processes, but a division is attempted herefor clarity.

3.1. transport mechanisms

3.1.1. Transport through a fluid: diffusional flow

Diffusional flow is the first of two basic transport mechanisms. This transport mechanismdoes not necessarily involve movement of a fluid: even in a completely stagnant fluid, therecan be a net flux of dissolved material through the fluid if there is a gradient in chemicalpotential of that dissolved material and the fluid provides a connected pathway. Diffusion is ageologically very important transport mechanism. It is the primary transport 'vehicle' fordissolution-precipitation creep (Durney & Ramsay 1973, Durney 1976, Raj 1982, Rutter1983) and metamorphic reactions. Although diffusion is a very effective transport mechanismon the small scale (< cm-dm), it is relatively ineffective for transport over larger distances.

3.1.2. Transport with a fluid: fluid flow

The second basic transport mechanism is fluid flow. When a fluid flows, it takes with it itssolute. Aqueous fluid have a very low viscosity (compared to rocks) and can therefore moveeasily and quickly over large distances through rocks. Fluid flow is therefore the onlyeffective mechanism for transport of dissolved material over large distances (>m-km) throughrocks (see Jamtveit & Yardley (1997) for a recent review). Again, we can distinguish twotypes of fluid flow (Appendix A):

1) fluid flow through channels (e.g. fractures) or a permeable medium (Darcian or advectiveflow);

2) fluid flow with its containing fracture (mobile hydrofractures).

3.1.3. Darcian or advective fluid flow

In the first case, Darcian or advective flow, fluid flows down a gradient in hydraulic head,through interconnected pathways. These pathways can be distinct macroscopic channels, suchas fractures, or the pores inside a solid permeable rock (pervasive flow). With localised orchannellised flow, the fluid by-passes most of the rock volume, whereas with pervasive flow,the fluid comes in close contact with most of the rock. This of course has significantimplications for the chemical interaction between fluid and rock (Rye & Bradbury 1988).

3.1.4. Mobile hydrofractures

In the second case, fluid is contained as a unit in a fracture and both fracture and fluid moveat the same time and the same rate. This transport mechanism is known as hydrofracturemobility (Weertman 1971, Secor & Pollard 1975, Pollard 1977, Takada 1990) and is invokedto explain the rapid ascent of magmas (Clemens & Mawer 1992, Clemens 1998), but hasreceived relatively little attention in metamorphic hydrology or hard-rock structural geology.Transport rates in case of hydrofracturing are not determined by Darcy's law. Transport isvery rapid (in the order of metres per second), but intermittent: short bursts of mobility arefollowed by possibly long periods of stagnation (or other types of flow). Hydrofracturemobility is a mechanism for rapid long distance transport of fluids, without much interactionwith the wall rock during transport (Davies 1999, Bons in press b)

The difference in rate controlling factors between Darcian flow and flow by mobilehydrofractures can be illustrated by the analogy of a gardener watering his plants. TheDarcian flow case would be the case where the gardener uses a hose. The rate at which waterreaches the plants is determined by the water pressure at the tap, the diameter of the hose andthe length of the hose. The gardener using a bucket to carry the water would be analogous tomobile hydrofracture transport. The rate is now determined by the rate at which the bucketcan be filled, the size of the bucket and the distance and walking speed of the gardener. 

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3.2 Precipitation in a vein3.2.1. The crack-seal mechanism

Before discussing the processes and causes for precipitation inside a vein, it is worthwhile to brieflydiscuss the crack-seal mechanism. The crack-seal mechanism was first introduced by Ramsay (1980) andhas quickly become the accepted mechanism for the formation of fibrous/elongate blocky veins (Cox etal. 1986, Urai et al. 1991, Kirschner et al. 1993). The mechanism itself does not dictate any fluid or solutetransport mechanism, nor a specific cause for precipitation inside a vein. The essence of the crack-sealmechanism is the repeated formation of a crack which is subsequently sealed by precipitation of veinforming material inside that crack. The crack-seal cycle can be repeated hundreds of times, typicallyadding about 10 µm to a vein each cycle (Fisher & Brantley 1992). The crack-seal mechanism can explainmany parallel wall rock inclusion trails that are often found in veins (Fig. 13 & 21). The effect of thecrack-seal mechanism on the morphology of vein forming crystals (stretched / elongate blocky / fibrous)and the tracking-capability of elongate crystals is further discussed in Appendix B.

3.2.2. Vein growth without fracturing

Probably the most clear-cut case of vein growth without fluid flow, but by diffusional solute transport is apressure fringe adjacent to a rigid object (e.g. a pyrite grain). The presence of a rigid object in adeforming/stressed rock causes a perturbation of the stress and pressure field around the object (Durney &Ramsay 1973, Strömgård 1973, Durney 1976, Selkman 1983). The sides of the object facing the highestcompressive normal stress experience the highest pressure, while the sides facing the lowest normal stressexperience a relative low pressure (Fig. 24). This pressure difference can drive dissolution at the highpressure sides, diffusional transport to the low pressure sides and precipitation there to form the pressurefringes. This process can occur without the formation of a (thin) crack at the low pressure sides of therigid object. Pressure fringes are usually fibrous and generally have no wall rock inclusions and thereforeshow no typical indicators for repeated crack-sealing. However, crack-sealing can occur, as has beenshown by Lister et al. (1986) for biotite porphyroblasts where radiator structures formed in pressureshadows.  

Figure 24. Flow lines (arrow lines) andpressure contours around a spherical object ina homogeneous linear viscous matrixdeforming in horizontal dextral simple shear.Far away from the object the principle stressesare at 45° to the flow plane. If the far fieldpressure is P, then σ1=P+∆σ and σ3=P-∆σ andthe differential stress is 2∆σ. The maximumstress perturbations occur on the surface of therotating sphere, where stresses range fromP-2∆σ (blue) to P+2∆σ (red). The differencein pressure can drive diffusional materialtransport from the red quadrants to the blueones, where precipitation leads to the growthof pressure shadows (distributed precipitation)or pressure fringes (localised precipitation).The flow field and pressure contours werecalculated with equations from (Chwang &Wu 1975), modified for simple shear.

It may be important that pressure fringes are usually fibrous (not elongate blocky) and appear to formwithout any brittle failure: there are no microstructural indicators for fracturing and the stress and pressure

field around the object provide the driving force for material transport and precipitation in the fringe. Thissupports the hypothesis that the fibrous crystal morphology may form in the absence of fracturing, as wasalso proposed by Taber (1918) and Mügge (1928), and more recently by Durney & Ramsay (1973),Janssen & Bons (1996) and Bons & Jessell (1997). In the absence of fractures, diffusion is the only viabletransport mechanism and we may infer that fibrous veins are indicative of diffusional material transport. 

Taber (1918) argued that the base of fibrous crystals must be in contact with a supersaturated solution inpores in the wall rock. Fibres "grow out of" the pores, which means accretion of new material is at thewall rock - vein interface. The work of Taber has recently been replicated by Means and Li (1995a,b) andLi and Means (1995), who produced very natural looking fibrous textures, without any fracturing. Mügge(1928) argued that the absence of growth competition in fibrous aggregates indicates the absence of anopen fracture in which the crystals grew. He delineated three cases:

(1) The opening rate of the vein (fracture) is faster than the maximum potential growth rate (on fastestgrowing crystal faces). Euhedral crusts of crystals form.

(2) The opening rate of the vein (fracture) is slower than the maximum, but faster than the minimumpotential growth rate of crystals. Growth is partly constrained and elongate-blocky crystals would formwith a crystallographic preferred orientation (CPO).

(3) The opening rate of the vein (fracture) is slower than the minimum potential growth rate. The surfacesof the growing crystals remain in contact with the opposite wall rock at all times and growth competitionis therefore inhibited. Fibrous crystals form without the development of a CPO.

Pabst (1931) measured c-axes orientations in fibrous pressure shadows to test the hypothesis of Mügge(1928) and indeed found that the fibres did not develop any CPO. The stretched and elongate-blockyquartz veins of Cox (1987), on the other hand, did show a CPO, which is in line with Mügge’s (1928)hypothesis that such veins fall into category (2), with the refinement by Ramsay (1980) that opening ratesare not constant, but that space is created in distinct steps by repeated crack-events. 

In the case of fibrous growth in pressure fringes, the driving forces for diffusional transport andprecipitation are evident: the stress and pressure perturbation caused by the relatively rigid object. Cangrowth without fracturing in other fibrous vein types be explained, when the transport mechanism andcause for precipitation is less evident? Well developed antitaxial fibrous calcite veins in shales are foundin Opaminda Creek (northern Flinders Ranges, South Australia). The Young’s modulus of calcite is atleast about double that of shale (Birch 1966). An isolated (lenticular) calcite vein therefore forms a hardobject relative to the surrounding shale and, during deformation, would (as for rigid pyrite grains) developa pressure gradient around itself (Fig. 25), which can drive further growth of the vein. Such veins can thenbe regarded as ‘self-supporting’ pressure fringes. Propagation at the vein tips, where a strongly localisedstress perturbation occurs, lengthens the vein as it widens, which probably enhances the process.  

Figure 25. Pressurecontours fordeformation of alenticular inclusion in alinear viscous matrix.The model is shown onthe left with theimposed verticalshortening boundaryconditions (arrows).Contours for pressureare shown on the rightfor the whole model(yellow square) and forthe area around the

inclusion tip (orangesquare). (a) Pressurecontours fordeformation around ahard inclusion(viscosity 100 x that ofmatrix). Pressure insidethe hard lens is high,but the highest pressureis found at the inclusiontips. Lowest pressure isfound at the sides of theinclusion. (b) Pressurecontours for a softinclusion (viscosity0.01 x that of matrix),simulating an openfracture. Lowestpressure occurs insidethe inclusion andhighest pressures at thesides of the inclusion.Except for thesingularity at the tip ofthe hard inclusion, thesolutions for hard andsoft inclusion are theinverse of each other.(c) Pressure profilealong paths A-B andA-C for a hardinclusion. Any transportdown the pressuregradient would bringmaterial from theinclusion tip area (C),but also from the farfield (B) to the surfaceof the inclusion. Thiscould explain thegrowth of antitaxialveins withoutfracturing. (d) Pressureprofile along profileA-B for a soft inclusionor fracture. The highestpressure occurs adjacentto the fracture, whichwould be the site ofmaximum dissolution ifdissolution-precipitationcreep operates. Materialtransport could betowards the vein, butpossibly also away from

 

the vein, down thepressure gradient.Pressures werecalculated with thefinite-element packageBASIL (Barr &Houseman 1992, Bonset al. 1997).

For this process to work, a ’seed’ vein must already be present. Many of the antitaxial fibrous veins atOpaminda Creek have a median zone that is actually blocky in texture and fibrous crystals startedgrowing from the surface of this zone (Fig. 19). The veins may have thus started their life by anotherprocess than described above and subsequently grew wider and longer by material transfer andprecipitation by a self-supported pressure gradient. It should also be noted that fibrous veins at OpamindaCreek occur as isolated lenses, but also as long (metres) and thin (cm’s) fracture-shaped veins, both ofwhich are antitaxial and fibrous. The shape of these veins is probably related to fracturing, which mayhave only seeded these veins. However, the actual driving force for subsequent fibrous growth infracture-shaped veins is not known.

3.2.3. Vein growth in fractures

Fractures are the most common sites for veins to form, as fractures provide space for precipitation andpreferred pathways for fluids to flow through. Two main causes for precipitation inside fractures can bedistinguished:

a) Vein-forming material is derived from a fluid that resides in both fractures and surrounding wall rock.The conditions inside the fracture and in the wall rock are different, such that a fluid that does not produceany (significant) precipitation in the wall rock does precipitate one or more minerals inside the fracture.Vein formation can then occur in the fracture by either diffusional transport in a possibly stagnant fluid orby flow of a fluid that brings local fluid from the wall rock into a vein. Again we can distinguish twocases:

a1) The fluid is supersaturated with respect to the vein forming mineral(s) in both fracture and wallrock, but precipitation is inhibited in the wall rock. Taber (1918) and Putnis et al. (1995) arguedthat a low porosity can inhibit precipitation and therefore a pore fluid can remain significantlysupersaturated until a fracture provides a possible site for precipitation. Material transport to suchfractures can be by diffusional transport (Fig. 26.a) or by advective flow. 

a2) The difference in conditions between fracture and wall rock cause a difference in the chemistryof the fluid residing in fracture and wall rock (Fig. 26.b). This could, for instance, be due to adifference in fluid pressure in both reservoirs. Silica solubility is pressure dependent, and therefore,we may expect that at a constant concentration of silica, fluid may be undersaturated in a high fluidpressure wall rock and supersaturated in a low fluid pressure fracture, where precipitation thenoccurs. Veins are often associated with deformation by dissolution-precipitation creep or ‘pressuresolution’ (Durney 1976, Rutter 1976; 1983, Raj 1982, Lehner 1990). This deformation mechanisminvolves dissolution at certain sites, transport of dissolved material and precipitation of that materialat other sites. The redistribution of material produces a change of shape of the rock: deformation.Re-precipitation can occur pervasively in the rock, but can also be localised, in which case veinsform (Beach 1977, Winsor 1984, Cox et al. 1986, Fisher & Brantley 1992, Gratier et al. 1993) (Fig.27). 

Figure 26. Two possible cases ofdiffusional transport towards avein, illustrated by graphs ofchemical potential of dissolvedvein forming mineral (µ) againstdistance, x, from a vein. (a)Equilibrium µ (red line) is thesame in vein and wall rock, butprecipitation is inhibited in thewall rock. Precipitation in the veinis possible, which lowers theactual value of µ in the vein. Thisproduces a gradient in actual µ(blue line), driving transporttowards the vein. (b) Equilibriumµ is lower in the vein than in thewall rock (red line). This causes agradient in actual µ (blue line),with again precipitation onlyinside the vein. 

Figure 27. Stylolite and quartz vein combination in Heavitree Quartzite(Ormiston Gorge, Central Australia). A distinct stylolite dips shallowlyto the right and truncates milky white vertical quartz veins. The jaggedsurface of the stylolite, common in limestones but relatively rare inquartzites, indicates vertical shortening, consistent with the horizontalstretching that is indicated by the quartz veins. Scale bar is 1 cm.Photograph courtesy Alice Post.

 

b) Vein forming material is derived from an extraneous fluid that enters the fractures, where it is/becomessupersaturated with respect to the vein forming mineral(s). Fractures, of course, provide high conductivitypathways for channellised fluid flow. The presence of fractures thus allows fluid to flow over longdistances and, in the process, become over- or undersaturated in its solute, primarily due to changes inpressure and temperature. 

Minerals precipitating in a fluid filled fracture grow into an open fluid, even though there may only be afew microns of free space, before the other side of the fracture is encountered. The shape of the crystals isthen determined by the growth competition of different minerals and the competition between crystalswith different crystallographic orientations. The resulting texture in the vein depends on several factors(Urai et al. 1991):

- the morphology of the fracture surface (smooth, rough);

- the width of the fracture;

- the growth habit of the vein forming mineral(s).

The effects of the different factors are discussed and illustrated in more detail in Appendix B. Microscopicmorphologies that result from crack-sealing are typically elongate blocky and stretched crystals. The firstform when repeated fracturing occurs on the same fracture surface, while the second is the result ofrepeated fracturing along different surfaces.

3.2.4. Coupled fracturing and fluid flow

High fluid pressure, close to lithostatic, can only form in a low permeability regime which prevents rapid

drainage of the overpressured fluid (Sibson et al. 1975, Nur & Walder 1992). Close-to-lithostatic fluidpressures therefore are generally found only at deeper levels in the crust, roughly below the brittle uppercrust. As mentioned above, high fluid pressure can create high permeability through fracturing -especially when helped by deformation (Cosgrove 1993). The ensuing high permeability allows drainingof fluid, which brings the fluid pressure down towards hydrostatic levels. Closure of dilatant fractures andsealing of fractures by precipitate reduces permeability and fluid pressures eventually rise again. Thiscyclical behaviour of rising fluid pressure until sudden fracturing, release of fluid ("burping") andreduction of fluid pressure is known as "fault valve behaviour" or "seismic pumping" (Sibson et al. 1975,Nur & Walder 1992, Thompson 1997, O'Hara & Haak 1992) (Fig. 28). It is generally used to explainextensive veining, such as the gold-bearing quartz veins in mesothermal gold deposits (e.g. Victorian andYilgarn gold fields in Australia (Cox et al. 1986, Sibson & Scott 1998)). 

Figure 28. Movie illustrating the effectof an increase in fluid pressure (dial onright), which brings the Mohr-circle tothe point of tensile failure. Drainage ofthe fluid through the newly createdfracture, causes a decrease in fluidpressure again, bringing the situationback to the start of the cycle.

Changes in temperature (Eisenlohr et al. 1989) and pressure (Vrolijk 1987, Henderson & McCaig 1996)are the primary causes for precipitation of vein forming material from a fluid that is flowing throughfractures. Chemical interaction with the wall rock and mixing of different fluids are other possible causes(Boullier et al. 1994), but as veins occur in almost any type of rock, these can not be the main reason forprecipitation. As quartz is the main mineral in large fracture hosted vein sets, the discussion belowfocuses on the solubility of silica. Silica solubility is primarily dependent on temperature (Fig. 29), exceptat high temperature and low pressure, where the solubility becomes strongly pressure-dependent. Thesolubility of silica is about 500 ppm in pure water that is at a lithostatic pressure of about 200 Mpa atabout 250°C (consistent with a depth between 5 and 10 km and a geothermal gradient in the order of30°C/km, point A in Fig. 29). Dropping the fluid pressure to hydrostatic pressure (point B) reduces thesolubility to about 400 ppm. The drop in pressure thus causes about 20% of dissolved quartz to precipitateand 10,000 kg of water is needed for every kg of vein quartz. The pressure sensitivity is even smaller atlower temperature. The large amount of fluid needed (fluid/rock ratio about 10,000 and at least 1000) is aproblem. One source for the fluid can be metamorphic dewatering at deeper levels. Another possibility isrecycling of water by pumping fluid in and out of the rock ("seismic pumping", Sibson et al. 1975) orconvective flow, that allows the fluid multiple passes through the same veins (Etheridge et al. 1983;1984).  

Figure 29. Contours of silica solubility(in ppm) in pure water as a function ofpressure (P in kbar) and temperature (Tin °C). Solubility is stronglytemperature dependent, with a strongpressure dependency only in the low Pand high T domain. PT-lines are drawnfor geothermal gradients of 20, 30 and40°C for a lithostatic pressure-depthgradient (brown, ρ=2.75·103 kg/m3)and a hydrostatic pressure-depthgradient (blue, ρ=1.0·103 kg/m3).Contours are based on the equation for

silica solubility as a function oftemperature and specific volume ofpure water (Fournier & Potter 1982).Specific volume values are fromtabulated data in Bowers (1995) (<2kbar) or calculated with equationsprovided by Kerrick & Jacobs (1981)(>2 kbar). Reduction of fluid pressure(Pf) from lithostatic (point A) tohydrostatic (point B) leads to about100 ppm (~20%) reduction insolubility at 250 °C and a geothermalgradient of 30 °C/km. Hotmetamorphic fluid (point C) has adramatically higher solubility (~10,000ppm). Most silica would precipitateclose to the source during slow ascent(path I) (Connolly 1997). Rapid mobilehydrofracture ascent (path II) wouldsee silica precipitation much higher inthe crust.

Metamorphic dewatering produces hot water at depth. Solubility of silica at source conditions of the fluidare high, in the order of >10,000 ppm (point C in Fig. 29). Cooling of the fluid as it flows upwards causesalmost all of the dissolved quartz to precipitate at deep levels (Connolly 1997). One would thus expectmost quartz veins to form just above the level of dewatering. However, extensive quartz veining is usuallyfound at much shallower levels. It seems that metamorphic dewatering and normal advective (Darcian)upward flow of fluid cannot explain the formation of extensive vein sets at shallow levels: more than 90%of the dissolved quartz would have precipitated before reaching the veining level as flow is too slow tomaintain any significant elevated fluid temperature. 

Metamorphic dewatering can, however, produce extensive quartz veining at or just above thebrittle-ductile transition, without significant quartz precipitation at deeper levels, if the fluid can rise fastenough not too cool (much) before reaching the brittle-ductile transition. Mobile hydrofracturing providesthe mechanism for such a rapid rise (Appendix A). The rise of water filled fractures at rates in the order ofmetres per second allows the water to stay hot until arrest in brittle levels of the crust (Bons in press b).Upon arrest (point A & B in Fig. 29), the water then still contains several thousands ppm of silica, whichis immediately dumped as the water cools to the much lower ambient temperature. As is explained inAppendix A, arrest occurs in the base of the brittle region, where the fluid can enter pre-existing fracturesor where a low (hydrostatic) fluid pressure causes leakage of the fluid into the wall rock. In the latter case,one can expect significant wall rock alteration to occur. 

Shear zones on small scales to crustal scales focus fluid flow, due to their enhanced permeability, eitheron the microscopic scale (dilatant grain boundaries or microfractures) or the macroscopic scale (fractures)(Connolly 1997, Thompson 1997). Many shear zones show evidence of enhanced fluid flow in the formof retrograde mineral reactions that involve hydration and/or the presence of veins (Rutter & Brodie 1985,Dipple & Ferry 1992, O'Hara & Haak 1992). Shear zones are also the preferred pathways for mobilehydrofractures, as they provide a relatively weak zone for hydrofracture propagation and, in case ofcrustal scale shear zones, provide a continuous pathway through the crust for ascending fluids. Manyextensive vein systems are therefore associated with major fault and shear zone systems (Eisenlohr et al.1989).

Veined fractures are usually regarded as the product of fluid flow. Hence, their study can provide insightin how fluid flows. This view should be reconsidered: veins that form from mobile hydrofracture arrestmay tell us more about how fluid failed to flow. This philosophical point can perhaps best be explained

with the analogy with granites: the shape and structures of granite bodies tell us most about theemplacement mechanism (stoping, ballooning, etc.) and much less about the mode of transport of granitemelt (Clemens 1998). 

Significant supersaturation, open fluid-filled fracture space and repeated fracturing are characteristic forboth veins that form from extensive flow through fracture networks and by arrest of mobilehydrofractures. Elongate blocky textures and evidence for crack sealing can be expected. Significantsupersaturation can cause ongoing nucleation of precipitating minerals, which produces a blocky texture. 

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4 Conclusions

 

Veins can have a range of microscopic morphologies. Even a single vein can have differenttextures, such as polytextured blocky + fibrous veins (Fig. 14). There is not a clear one-to-onerelationship between macroscopic and microscopic morphology. Fibrous crystals can befound in pressure fringes, lenticular veins and fracture-shaped veins. It is therefore not easy tomake a simple link between morphology and conditions and processes of vein growth. Also,the processes that lead to vein formation are not sharply delineated. For instance, duringseismic pumping, a single growing vein may experience periods of advective fluid flowbetween periods of dominantly diffusional nutrient transport. Such a variation in process canalso occur in space: while at one spot fluid may flow through a fracture and deposit veinmaterial, another fracture nearby may be dead-ended and not experiencing fluid flow, butonly vein growth by diffusional nutrient transport. Despite this, an attempt can be made tocorrelate processes and vein morphology as much as possible (Fig. 30).  

Figure 30. Schematicrepresentation of therelationship of veinmorphology, vein growth raterelative to dilation rate, andnutrient transport mode.Graphs on the vertical axisshow the wall rock dilation(red line) and the width of thevein (blue line) as a functionof time.

As was discussed in this paper, fibrous veins may form without fracturing. This implies thatnutrient transport is either by diffusional or by pervasive porous flow transport. Consideringthat pressure fringes are usually fibrous and are in their shape controlled by the local stressdistribution, diffusion is the most probable nutrient transport mechanism for pressure fringesand therefore for fibrous vein growth. In case of pyrite-type (antitaxial) pressure fringes, thesite of lowest pressure is at the surface of the object, which is the growth plane. In case ofcrinoid-type pressure fringes and solid relatively rigid veins, the lowest pressure lies on theoutside of the fringe or vein, which is indeed the growth plane (antitaxial veins). Infracture-less growth, the rate of vein growth is at all times exactly the same as the dilationrate, which is the rate at which the wall rock on either side of the vein moves apart.

An important class of veins is formed by those veins that formed by repeated crack-sealing.In that case, the dilation rate is spiky, with the vein growth rate passively following to fill anydilation. Elongate blocky textures form when the growth plane remains a plane of weakness(localised fracturing), possibly due to imperfect sealing. Such veins are in principle syntaxial.If fracturing occurs at different sites in a vein throughout the dilation history (distributedfracturing), and thus cuts through previously grown vein material, stretched veins form. Bothelongate blocky and stretched crystals form in the absence of ongoing nucleation, which

precludes large supersaturation in the vein forming fluid. The nutrient transport mechanismcan be diffusion and/or advective flow.

An interesting question is whether crack-sealing and fractureless vein formation are endmembers of a continuous spectrum or distinct processes. Macroscopic fractures and grainboundaries are distinctly different, but microcracks and wide grain boundaries as described byDrury & Urai (1990) may provide the link between the two end members. This questionremains open.

Sudden extensive dilation leads to the formation of large cavities. Such cavities can only format depth by excessive fluid pressures, which can arise from seismic pumping / fault-valve likeprocesses or by the arrival and arrest of batches of fluid from rising mobile hydrofractures.Large cavity-fill veins usually have a blocky texture, indicating large supersaturation.Extremely large supersaturations can occur in fluids that very rapidly rose through the crust inmobile hydrofractures. The veins that formed from such fluids indicate arrest and suddenchemical equilibration of these fluids, not their passage.

In recent years, much emphasis has been put on crack-sealing and advective fluid flow.Although these processes are undoubtedly very important for the formation of veins, otherprocesses should not be neglected. Fibrous veins and pressure fringes that formed byfracture-less vein growth are important for strain analysis, while mobile hydrofracturesprovide a mechanism to rapidly transport vein forming minerals (particularly quartz) and,more importantly, ore minerals such as gold, up through the crust.

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Acknowledgments

 

Without wanting to insult anyone else, I would like to say that Win Means has probably beenthe most inspiring scientist for me. His help and encouragement for my research are gratefullyacknowledged. My study of veins would not have led anywhere without the guidance andmany discussions with Mark Jessell and Janos Urai. Nick Oliver is thanked for introducingme to many of localities with fascinating veins and he, Alvar Soesoo, Marlina Elburg, ChrisJanka, Michelle Robinson and others are thanked for their company and help in the field.Lynn Evans is thanked for her help with the numerical modelling with BASIL. Margaret,Doug, Griselda and the late Reg Sprigg are thanked for their hospitality and access to rocks atArkaroola, as are Maurice and Jane Phillips for Poolamacca. Anne-Marie Boullier and JanosUrai are thanked for their careful review of the manuscript. Research was initially fundedthrough an ARC Postdoctoral Fellowship and was subsequently funded by MonashUniversity, through a Logan Research fellowship. Finally I would like to thank Mark Jesselland Janos Urai for giving me the honour to publish in this CD-amicorum.

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Appendix A. Darcian flow and mobile HydrofracturesThe basics of Darcian or advective flow through a permeable medium are described in Darcy’sequation for one-dimensional flow:

Q = - A · K · ∆φ/∆x (A.1)

This equation states that the flow rate, Q (m3 s-1), along a line in the x-direction, is linearlyproportional to the cross-sectional area, A (m2), through which the fluid flows, the hydraulicconductivity, K (m s-1), and the gradient in hydraulic head, ∆φ/∆x (m/m). A more generalthree-dimensional form of Darcy’s equation is:

q = - K · δφ/δx (A.2)

where the vector q (m s-1) is the rate of flow, per cross-sectional unit area, in the direction x.

Hydraulic head is often confused with pressure when it is stated that "fluid flows from high to lowpressure". Obviously, it is not so simple as water does not flow from the bottom of a glass (highestP) to the surface (lowest P). The hydraulic head (unit is metre) is given by:

φ = P / ρg + z (A.3)

The first part of the equation is the pressure head, determined by the pressure, P (Pa), the fluiddensity, ρ (kg m-3), and the gravitational acceleration constant, g (m s-2). The second term, z (m), isthe elevation head, which is simply the elevation above some datum level. If the pressure is simplythe hydrostatic pressure, then the hydraulic head is the same at any level. If fluid pressure increasewith depth is equal to the lithostatic pressure gradient (say 2.75·104 Pa/m), then the hydraulic headgradient is 1.75 and fluid would flow upwards (Fig. A1).  

Figure A1.Graphshowinglithostatic (redline) andhydrostatic(blue line)pressure-depthgradients inpressure-depthspace,assumingspecificdensities of2.7·103 kg/m3

for rock and1.0·103 kg/m3

for water anda gravitationalaccelerationconstant of 10

 

m/s2.Hydraulicheads arecalculated forpoints A to D,showing thatthe hydraulichead gradientis 0 (no flow)for thehydrostaticpressuregradient and-1.75 for thelithostaticcase, whichresults inupwards fluidflow.

The hydraulic conductivity inside an open fluid reservoir is infinite and hence the hydraulic headinside a water-filled fracture is the same at any level in that fracture. As a consequence, the pressureinside such a fracture increases by about 104 Pa/m with depth. Suppose now that we have a verticalfracture of length 2L that is filled with water (ρ~1·103 kg/m3) in a surrounding rock with a densityof 2.7·103 kg/m3 (Fig. A2). It is clear that the vertical pressure gradient inside the fracture (1.0·104

Pa/m) is different from the pressure gradient in the adjacent rock (2.7·104 Pa/m). Suppose now thatthe pressures balance halfway the fracture, which level we take as a reference (z=0 m). At the uppertip of the fracture, the fluid pressure exceeds the rock pressure by L · 1.7·104 Pa. The fluid isunderpressured with respect to the rock pressure by the same amount at the bottom end of thefracture. This puts a limit on the length of an open fluid-filled fracture. Above some critical length,the overpressure at the upper end of the fracture causes the rock to further split open, while theunderpressure causes the fracture to be closed at the bottom end. If the fluid volume is to remainconstant, opening and closure must occur in tandem and when this happens the fracture movesupwards together with its fluid. 

Figure A2. Pressurestate inside a verticalfluid filled fracture,shown in apressure-depth graph.The density differencebetween fluid (1.0·103

kg/m3) and rock(2.7·103 kg/m3) causesoverpressure at theupper end of thefracture andunderpressure at thelower end. Over- andunderpressure are

proportional to thelength (2L) of thefracture. 

Secor & Pollard (1975) provided a simple equation to estimate the onset mobility of a verticalfracture:

Lmax = 1.36 · (Kc/S)2/3 (A.4)

Lmax is the maximum length of a stable fracture, Kc is the fracture toughness and S is the effective

normal stress gradient (S = δρ / δz, ~ 0.015 MPa/m). The fracture toughness is about 1-3 Mpa·m1/2

for intact crystalline rocks (granite, gneiss; Secor & Pollard 1975), but is probably lower for schistsand low grade rocks, especially for fracturing parallel to schistosity. One should note that S is givenhere for a vertical fracture. The value of S is smaller for an inclined fracture, but can be significantlyhigher when a fracture is subject to a tectonic stress gradient, in which case S can possibly reachvalues of 0.1-1 MPa/m.

Figure A3 shows that vertical water filled fractures can reach lengths of about 5-100 m. Above thatlength, such fractures become unstable and rise rapidly, in the order of metres per second. A mobilefracture can thus easily ascend through the crust in a matter of hours. A mobile hydrofracture stopswhen the conditions of equation (A.4) are no longer satisfied. This can be because of rock type witha higher fracture toughness is encountered or when S decreases. The latter is probably the mainreason for arrest. At any time it is easier for a fluid to enter pre-existing fractures than to propagate afracture through intact rock. The presence of fractures thus causes the dispersal of fluid into thesefractures, lowering S. For a hydrofracture to be mobile, the average pressure in the fluid must belithostatic. Fluid is lost to the wall rock when a fracture reaches lower than lithostatic fluid pressuresin the wall rock. Low fluid pressures and pre-existing fractures occur above the brittle-ductiletransition and hence mobile hydrofractures are to be expected to get arrested at this level.  

Figure A3.Graph ofcritical verticallength as afunction ofeffectivenormal stressgradient (S)(after Secor &Pollard 1975).For fracturetoughnessesbetween 0.1 and10 Mpa·m1/2,water-filledfractures areunstable due totheir buoyancyabove 5 to 100m length. Forcomparison,granitemagma-filledfracturesbecomeunstable whenlonger than 20to 300 m.Tectonic stressgradients cangive rise tomuch higherS-values 

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Appendix B. Crack-Seal microstructuresThe shape and orientation of elongate blocky and fibrous veins can provide information aboutthe opening trajectory of a vein. The opening trajectory is the movement path taken by twoparticles that originally were adjacent to each other on either side of the vein. Elongate and, inparticular, fibrous crystals can be curved and often seem to follow the opening trajectory(Durney & Ramsay 1973). However, crystals can become curved due to deformation(Williams & Urai 1987) or may not follow the opening trajectory in the first place (Urai et al.1991). Urai et al. (1991) developed a model to explain the not always perfect "trackingcapability" of vein crystals that form during repeated crack-sealing. Each time a fractureforms, the crystal tips start growing outward into the fracture space. The direction of growthis generally perpendicular to the local fracture surface. If the fracture is rough or wavy, thisgrowth direction may not be parallel the opening trajectory. The result is that grainboundaries have a tendency to converge on protrusions and ridges on the fracture surface(Fig. B1). If the ridges are distinct and sharp, they may lock a grain boundary and thisboundary will subsequently follow the ridge and hence the opening trajectory. If the fractureis smooth, grain boundaries do not get locked or to a lesser extent, and grain boundaries thendo not or only partially follow the opening trajectory.  

Figure B1. Movie of grains growing by repeatedcrack-sealing. Growth is isotropic as envisaged in themodel of Urai et al. (1991). Grains grow outward fromthe surface and thus converge on each other inembayments between ridges. Eventually, only grainboundaries are left that are growing vertically towardsa ridge on the opposite side of the crack. 

To further investigate the model of Urai et al. (1991), a computer program was written tosimulate the crack-seal process. The model is only briefly described here, and the reader isreferred to the full description of the model and a first systematic investigation with the modelthat are given by Bons (in press a) and Hilgers et al. (in press). In the 2-dimensional model,grains are defined by nodes that are linked by straight boundary segments. An initialhorizontal surface fracture is created with a user-defined roughness. Every N time steps, thelower fracture surface is moved a user-defined distance and direction to simulate acrack-opening event. All grain surfaces that are then exposed to the open fracture growoutward by repeatedly moving the boundary segments over small distances one at a time. Thesegments are moved until they reach the other side of the fracture space (Fig. B2). The rate ofgrowth is a function of the angle (α) between the boundary segment and the c-axis of a grain,which defines the lattice orientation. The type of function defines the growth habit of thecrystal and therefore different "minerals" can be defined. In the examples below, two"minerals" were used:

"Square mineral". The growth rate is fastest in the directions parallel and perpendicularto the c-axis. The habit of this mineral is that of a square.

●

"Prismatic mineral". Growth is fastest parallel to the c-axis and slowest normal to it. Asecondary growth rate minimum occurs at 30° to the c-axis, which gives the mineral a

●

 

quartz-like prismatic habit.

Figure B2. Movie of crack-seal vein growth to illustrate theworking of the program VEINGROWTH. Each crack-event,the lower wall rock is moved, in this case down and to theright. Exposed surface segments of grains grow into thecrack. Their growth rate is determined by the relativeorientation of the surface segment and the c-axis of the grainand the growth habit of the "mineral", in this case theprismatic "mineral". The orientation of the c-axes is shownby the shading: darkest for horizontal and white for verticalc-axes. The crystallographic control on growth rate leads tothe development of faceted grain surfaces. However, if thecrack is too thin, such facets may never fully develop beforethe crack is sealed again. In this example faceting is justforming before sealing.

Growth in an open cavity

Growth in an open cavity best illustrates the effect of different mineral types on themorphology of the vein fill. Figure B3.a shows the result of 800 growth steps of the prismaticmineral into a wide open fracture with a rough surface. The shading of the grains is a functionof the c-axis orientation. One can see that the grains with a vertical c-axis (light) quicklyoutgrow differently oriented grains. Figure B3.b shows the same for 640 growth steps for thesquare mineral. Both grains with vertical (light) as well as with horizontal (dark) c-axes arenow the "winner" grains. 

a 

b 

Figure B3. Movies of simulated vein growth into anopen cavity. (a) Prismatic "mineral", growing for 800time steps. Grains are elongate blocky in shape (nonucleation, except at beginning on wall rock) withfaceted surfaces with the cavity. Growth is fastest inthe direction of the c-axis and minerals with verticalc-axes (light) outgrow other grains (dark). (b) Thesame for the square "mineral". Growth is fastestparallel and normal to the c-axes and both grains withhorizontal and vertical c-axes outgrow others. 

Crack-sealing with a vertical opening trajectory - effect of crack-width

Three movies (Fig. B4) show the effect of the crack width. Figure B4.a shows growth inmany (154), but small (2 pixels) vertical fracture opening events. Grain boundaries quicklyget locked on ridges, resulting in little reduction in average grain width as growth progresses.The spacing of the ridges determines the width of grains. If, keeping all other parameters the

same, the fracture opening is doubled to four pixels per event (Fig. B4.b), we see a decreasein locking capability of the ridges and a slightly larger average grain width developing.Increasing the fracture width to 16 pixels (Fig. B4.c) results in almost complete loss oflocking capability of ridges. These simulations confirm the model of Urai et al. (1991), thatthe locking capability of ridges is a function of the width of the fracture relative to theroughness of the fracture. The rougher a fracture and/or the thinner the fracture, the better isthe locking capability and hence the tracking capability of the vein crystals. The spacingbetween ridges determines the average crystal width when locking is strong. 

a

b

c 

Figure B4. Movies showing the effect of fracture width on the veintexture for the prismatic "mineral". All parameters are kept the same,except the vertical opening distance, which is (a) 2 pixels, (b) 4 pixelsand (c) 16 pixels. See text for discussion.

Crack-sealing with an oblique opening trajectory - effect of crack roughness

Figure B5 shows three movies of growth with an oblique (∆x=-2 & ∆y=4) opening direction,while all other parameters are kept the same except the roughness amplitude. The fracturesurface is too smooth in Fig. B5.a for any ridges to lock grain boundaries. The average grainwidth increases in the growth direction and there is no tracking whatsoever of the obliqueopening direction. Doubling the amplitude of the roughness (Fig. B5.b) leads to locking ofsome boundaries, but not all. Therefore, some boundaries follow the oblique openingtrajectory, while others grow vertically. The result is a specific microstructure with somestrongly elongate grains, some large blade like crystals and many "looser" grains (see Fig. 1.cin Bons & Jessell 1997). A further increase in surface roughness (Fig. B5.c) produces a betterlocking capability of the ridges and hence better tracking of the opening trajectory.  

Figure B5. Movies showingthe effect of fractureroughness on the vein texturefor the prismatic "mineral".Opening direction is 2 pixelsto the left and 4 pixels down.All parameters are kept thesame, except the fractureroughness, which varies from(a) smooth, (b) medium roughand (c) rough. See text fordiscussion.

Crack-sealing - change of opening direction

Finally, a case of growth with an abrupt change of opening direction is shown in Fig. B6. Thecase shown is for isotropic growth with a rough fracture surface, resulting in perfect trackingof the opening trajectory in the first growth period. After an abrupt change in openingdirection, perfect tracking is maintained, but several fibres get truncated soon after the changein direction. Such a sudden increase in average fibre width is often seen associated with achange in opening direction (Fig. 14.b). The change in opening direction causes a change inthe points where grain boundaries are locked. Temporary unlocking during the transitionperiod frees the grain boundary for lateral movement and possibly the truncation of the grain. 

Figure B6. Movie showing the effect of a change in opening directionfor an isotropically growing vein material. Opening direction is 2pixels to the right and 4 pixels down first and then 2 pixels to the leftand 4 pixels down. See text for discussion.

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